1. Field of the Invention
The present invention relates to an optical waveguide device used for optical information processing and optical measurement control performed utilizing a coherent light source, and a light source device and an optical apparatus including the optical waveguide device.
2. Description of the Related Art
In the fields of optical information recording and reproduction, a higher density of recording and reproduction is realized by using a light source for emitting light having a shorter wavelength. For example, whereas conventional compact disk apparatuses use near infrared light having a wavelength of about 780 nm, digital versatile disk (DVD) apparatuses for recording and reproducing information at a higher density use red semiconductor laser light having a wavelength of about 650 nm. In order to realize a next-generation optical disk apparatus for recording and reproducing information at a still higher density, development of blue laser light source devices have been actively developed. For example, a wavelength conversion element using a non-linear optical substance has been developed to be included in a compact and stable blue laser light source device.
FIG. 14 is a schematic view illustrating an exemplary blue light source device using a second harmonic generation element (hereinafter, referred to as an xe2x80x9cSHG elementxe2x80x9d) 117. With reference to FIG. 14, the SHG element 117 will be described.
The SHG element 117 includes a dielectric substrate 114 and a high refractive index area having a width of about 3 xcexcm and a depth of about 2 xcexcm formed by a proton exchange method. The high refractive index area acts as an optical waveguide 115. Infrared light emitted from a semiconductor laser 111 having a wavelength of about 850 nm is collected on an incident surface 139 of the SHG element 117 through a collection lens 112 and then propagated through the optical waveguide 115 in the SHG element 117 to form a fundamental guided wave.
Lithium niobate crystals forming the dielectric substrate 114 have a non-linear optical constant. As a result of sufficiently large non-linear optical constant, a harmonic guided wave having a wavelength of about 425 nm is obtained by wavelength conversion of the infrared light, and excited from the electric field of the fundamental guided wave.
In order to compensate for a propagation constant difference between the fundamental guided wave and the harmonic guided wave, domain invention areas 116 are periodically formed in the optical waveguide 115. The harmonic guided waves which are excited throughout the optical waveguide 115 are coherently added together and then come out from an outgoing surface 138 of the SHG element 117.
In order to correctly compensate for the propagation constant difference between the fundamental guided wave and the harmonic guided wave, the wavelength of the fundamental guided wave needs to be maintained at a certain value. Accordingly, as the semiconductor laser 111, a DBR laser is used for its very small wavelength fluctuation in accordance with the temperature or the like. A DBR laser has another feature in that since light is oscillated at a single wavelength, the light has a satisfactorily high coherency and a satisfactorily low RIN (relative intensity noise).
FIG. 15 is a schematic view of an optical disk pickup including the SHG element 117 shown in FIG. 14 for providing blue light. With reference to FIG. 15, an operation of the optical disk pickup will be described.
Harmonic blue light output by the SHG element 117 passes through a collimator lens 113, a polarization beam splitter 120, a 1/4 wave plate 121 and an objective lens 122 and then is collected to an optical disk 124.
The light modulated by the optical disk 124 is reflected by the polarization beam splitter 120 and guided to a light detector 125 by a collection lens 123. Thus, a reproduction signal is obtained.
The SHG element 117 outputs linearly polarized light in a direction parallel to the page. This light passes through the 1/4 wave plate 121 and returns through the 1/4 wave plate 121 to become a polarized light which is in a direction perpendicular to the page. Thus, the light reflected by the optical disk 124 is all reflected by the polarization beam splitter 120 and does not return toward the SHG element 117 theoretically.
However, the optical disk 124 includes a material having a birefringence. Accordingly, in actuality, an unnecessary polarized component returns toward the SHG element 117 through the polarization beam splitter 120.
While data stored in the optical disk 124 is reproduced, the objective lens 122 is positionally controlled to focus the light accurately to the optical disk 124. Accordingly, the outgoing surface 138 of the SHG element 117 and the optical disk 124 form a confocal optical system. Thus, the light reflected by the optical disk 124 is accurately collected at the optical waveguide 115 on the outgoing surface 138 of the SHG element 117.
In an optical system including a semiconductor laser as a light source, the light component which returns toward a light source after being reflected induces noise (mode hop noise). Conventionally, various proposals have been made for avoiding the mode hop noise.
For example, oscillation in a plurality of longitudinal modes is caused by modulating light from the semiconductor laser with a harmonic signal or by causing self-oscillation of the semiconductor laser.
In the field of optical communication, for collecting light from a semiconductor laser to an optical fiber, a light isolator utilizing a magneto-optical effect is commonly inserted between the semiconductor laser and the optical fiber.
Japanese Laid-Open Publication No. 5-323404 discloses a method, by which an incident surface of an optical fiber or an optical waveguide is obliquely polished, so that the returning light is obliquely reflected and does not return to the semiconductor laser.
These technologies are for reducing the mode hop noise induced by the light returning to inside the semiconductor laser as a light source.
The present inventors performed experiments on data reproduction by an optical pickup including the SHG element 117 shown in FIG. 15. As a result, the present inventors found a noise which is generated by the following mechanism, which is different from induction by the returning light.
The returning light collected at the optical waveguide 115 on the outgoing surface 138 of the SHG element 117 is reflected by the outgoing surface 138 and interferes with the light coming out from the optical waveguide 115. Thus, an interference noise is generated.
Due to such an interference noise, the optical power output from the SHG element 117 appears to have been changed from the optical disk 124, and thus a reproduction signal from the optical disk 124 is modulated with a low frequency noise, resulting in signal deterioration.
Whereas noise induced by the returning light is generated by the interaction of the light inside the semiconductor laser 111 and the returning light reflected by the incident surface 139 of the SHG element 117, the interference noise is generated by the interference of the light from the SHG element 117 and the returning light reflected by the outgoing surface 138 of the SHG element 117.
The present inventors found another cause of the interference noise as a result of a further research. A portion of the returning light from an external optical system external to the optical waveguide device (including, for example, collimator lens 113) is re-excited in the optical waveguide 115 as a guided wave and reflected by the incident surface 139 of the SHG element 117. The light reflected by the incident surface 139 is interfered with the light from the semiconductor laser 111. Such an interference also causes the interference noise.
As described above, an optical system including an optical waveguide device involves two different types of noises. One is a low frequency interference noise caused by the interference, in an external optical system, of (1) light emitted by the light source and propagating through the optical waveguide device toward the external optical system and (2) the light reflected by an outgoing surface or an incident surface of the optical waveguide device after propagating through the optical waveguide device and being reflected by an external object (e.g., the optical disk). The other is the mode hop noise caused inside the semiconductor laser.
Various proposals have been made in order to reduce the mode hop noise, but the interference noise caused in the external optical system has not been a target of attention and no proposals have been made for solving the problem of the interference noise.
According to the present invention, an optical waveguide device includes a dielectric substrate; and an optical waveguide formed in the dielectric substrate, the optical waveguide having a longitudinal axis and an outgoing surface disposed at an angle other than a right angle relative to a plane perpendicular to the longitudinal axis.
In one embodiment of the invention, the dielectric substrate has an outgoing surface disposed in a plane within which the outgoing surface of the optical waveguide is disposed.
In one embodiment of the invention, the outgoing surface is angled so that light going out from the optical waveguide in a first direction and reflected back to the outgoing surface by an external object is directed by the outgoing surface in a second direction different from the first direction.
In one embodiment of the invention, the optical waveguide has an incident surface disposed at an angle other than the right angle relative to the plane perpendicular to the longitudinal axis of the optical waveguide.
In one embodiment of the invention, the dielectric substrate has an incident surface disposed in a plane within which the incident surface of the optical waveguide is disposed.
In one embodiment of the invention, the dielectric substrate has an outgoing surface disposed in a plane within which the outgoing surface of the optical waveguide is disposed, and the incident surface of the dielectric substrate and the outgoing surface of the dielectric substrate are substantially parallel to each other.
In one embodiment of the invention, the dielectric substrate and the optical waveguide from a second harmonic generation element.
In one embodiment of the invention, the optical waveguide device further includes a reflection reducing layer for reducing a reflection of a harmonic wave.
In one embodiment of the invention, the reflection reducing layer is disposed on an incident surface of the optical waveguide.
In one embodiment of the invention, the optical waveguide device further includes a harmonic wave absorption element for absorbing a harmonic wave in a portion of the optical waveguide in the vicinity of an incident surface of the optical waveguide.
In one embodiment of the invention, the optical waveguide device further includes a grating element having a periodicity of xcex9 in a portion of the optical waveguide in the vicinity of the incident surface. The second harmonic generation element receives light having a wavelength of xcex as a fundamental wave in a vacuum; and the periodicity xcex9, an effective refractive index n of the optical waveguide, and the wavelength xcex fulfill the relationship of xcex/(4xc3x97n) less than xcex9 less than xcex/(2xc3x97n).
According to another aspect of the invention, a light source device includes any of the above-described optical waveguide devices and a collimator lens for substantially collimating outgoing light from the optical waveguide. The collimator lens is located at a center of distribution of the outgoing light from the optical waveguide.
According to still another aspect of the invention, an optical apparatus includes any of the above-described optical waveguide devices, and a collection optical system for collecting outgoing light from the optical waveguide on a target of detection. The optical waveguide device and the target of detection are located so as to be confocal with each other.
In one embodiment of the invention, an angle xcex8 made by a plane perpendicular to the outgoing surface and the optical waveguide, an effective refractive index n of the optical waveguide, and a substantial numerical aperture NA of a portion of the collection optical system which opposes the outgoing surface, fulfill the relationship of NAxe2x89xa6sin(xcex8xc3x97n).
In one embodiment of the invention, the target of detection includes an optical disk.
According to still another aspect of the invention, an optical waveguide device includes a dielectric substrate; and an optical waveguide formed in the dielectric substrate. Light going out from the optical waveguide in a first direction and reflected back to the outgoing surface by an external object is directed by the outgoing surface in a second direction different from the first direction.
In one embodiment of the invention, the dielectric substrate and the optical waveguide form a second harmonic generation element.
According to the present invention, a simple structure of an optical waveguide device, in which an outgoing surface is disposed at an angle other than a right angle with respect to a longitudinal axis of an optical waveguide, effectively reduces the influence of the light returning from an optical system external from the optical waveguide device (e.g., a pickup optical system).
In the embodiment in which the optical waveguide device has both an outgoing surface and an incident surface disposed at an angle other than a right angle with respect to a longitudinal axis of the optical waveguide, the reflection of the light by both surfaces is reduced and thus the influence of the interference of the light returning from the external optical system and the light propagating through and going out from the optical waveguide device is eliminated substantially completely. Since the incident surface is disposed at an angle as described above, the light emitted by a semiconductor laser and reflected by the incident surface is substantially inhibited toward returning from the semiconductor laser. Thus, the mode hop noise is also reduced as well as the interference noise.
In the embodiment in which the outgoing surface and the incident surface are parallel to each other, the optical waveguide devices are easy to produce.
Also according to the present invention, a light source device including the above-described optical waveguide device and an optical apparatus including such a light source are provided. A collimator lens in the light source device is located at the center of distribution of outgoing light from an optical waveguide of the optical waveguide device. Therefore, the light returning toward the light source device from the external optical system is reduced.
Due to the outgoing surface disposed at an angle as described above, light returning from an external optical system and reflected by the outgoing surface is prevented from interfering with the outgoing light from the optical waveguide. Therefore, a stable light source device with substantially no interference noise is provided.
In the embodiment in which an incident surface is disposed at an angle with respect to a longitudinal axis of the optical waveguide as well as the outgoing surface, the light returning from the external optical system is prevented from being reflected by the incident surface. Thus, the interference noise is further reduced. Furthermore, the light from a light source is prevented from returning toward the light source as a result of being reflected by the incident surface. Thus, the hop mode noise is also reduced.
Thus, the invention described herein makes possible the advantages of providing (1) an optical waveguide device for reducing an interference noise in an optical apparatus, and (2) a light source device and an optical apparatus including such an optical waveguide device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.